Ultraviolet (“UV”) light can be used as a means for disinfection. In particular, UV light from the sun can be used to disinfect fluids such as water. The intensity of UV light, exposure time, temperature, and cloudiness of the water are important variables to determine the rate of disinfection. SODIS is a procedure to disinfect contaminated fluid, typically drinking water, using solar radiation. Very generally stated, contaminated fluid is poured into a transparent container and exposed to the sun. Solar radiation, particularly UV radiation approximately in the wavelengths of 320-400 nm (UV-A and UV-B), inactivates pathogens (e.g. bacteria, viruses, and other microorganisms) in the fluid; in the case of water, rendering the water safe for human consumption. The solar radiation also heats the contaminated water, and temperatures above 40 degrees Celsius (C) enhance this process. Further, if the water is maintained above a certain temperature for a sufficient period of time, e.g. 60 degrees C. for 1 hour, the heat kills pathogens and renders the water safe for human consumption (solar pasteurization). Studies have shown that the process by which solar radiation eliminates pathogens is accelerated when the water is at higher temperatures. The presence of certain chemicals and compounds, such as photocatalysts (typically titanium oxide), photosensitizers, and dissolved oxygen in the water will also accelerate the SODIS process.
SODIS is advantageous over other forms of disinfection for a number of reasons. Principally, the process is more economical than boiling or chemically treating contaminated water. In many areas lacking adequate access to potable water, fuel for boiling can be quite expensive and chemicals for treating water prohibitively so, if available at all. SODIS, by contrast, only requires an ultraviolet transparent container (“UVTC”) and readily-available solar radiation, preferably strong sunlight. Additionally, SODIS is more environmentally friendly than other disinfection methods. Because the process uses only renewable solar energy, water can be disinfected without the use of fuel and without requiring any consumable media, such as chemicals or replacement filter media. Approximately five million people currently practice SODIS using polyethylene tetraphthalate (PET) beverage bottles.
Challenges With SODIS
Even though SODIS is conceptually simple, there are several practical challenges. These challenges generally reduce to knowing whether, when, and to what degree the process is underway and complete. Proper SODIS requires the right container, level of fluid cloudiness, and strength of solar radiation, which is a function of location and weather. Further, when faced with non-ideal conditions, such as cloud cover or weak sunlight, SODIS requires extended solar exposure and/or recognizing that SODIS is infeasible with a given set of conditions. Some factors, such as solar UV intensity, are unobservable with the naked eye. Further, it is impractical to manually measure and adjust for time-varying factors related to weather.
Given the practical challenges of SODIS, a set of general guidelines have been developed to increase the likelihood of conducting it correctly. It is typically recommended that water be exposed to direct sunlight for six (6) to eight (8) hours in good conditions, which include relatively clear water (<30 NTU, Nephelometric Turbidity Units), minimal cloud cover, and proximity to the equator. Contaminated water left outside for eight (8) hours during a period of moderate cloud cover may have received only a fraction of the radiation necessary to ensure disinfection. Thus, a person consuming the water could still become sick due to pathogens. In contrast, the disinfection process may move more quickly at high altitudes or nearer the equator where solar radiation doses tend to be higher, so that water may be disinfected more quickly than the time recommended by general guidelines.
In short, varying environmental and water quality factors can create uncertainty as to whether and to what degree the water has been adequately disinfected by solar radiation. This uncertainty prevents widespread use of an otherwise economical and environmentally-friendly method of treating contaminated drinking water. Thus, a system and method that correctly determines and indicates when the SODIS process is underway, to what degree SOIDS has progressed and whether it has completed, under a given set of conditions would greatly improve the dependability and consequently utility of the SODIS process. This would promote greater use of the SODIS process and allow for increased access to potable water.